The rotary drilling of a wellbore is accomplished by rotating a drill string having a drill pipe and a drill bit at its lower end. Weight is applied to the drill bit while it is rotating to create a borehole into the earth. The drill string is hollow and sections are added to the drill string to increase its length as the borehole is deepened. This rotary drilling process creates significant amounts of friction which produces heat along with fragments of the strata being penetrated. The fragments of the strata must be removed from the borehole and the drill bit must be cooled to extend its useful life. Both of these functions are accomplished by circulating a fluid down through the drill string and up to the surface between the drill string and the wall of the borehole. Generally, the borehole into which the casing or liner is introduced is filled with drilling mud.
While the use of manganese tetroxide (Mn3O4) in drilling fluid formulations is known, the use of such Mn3O4-containing formulations has generally been found to be damaging to the reservoir formation. Generally speaking, the permeability of reservoirs is reduced when they are contacted with drilling fluids and, as a result, the development of special and expensive stimulation techniques has proven necessary.
In horizontal wells, long-reached wells, and in maximum reservoir wells, the stimulation techniques which effectively remove material to restore permeability to the damaged reservoir are not possible since the fluid stimulation acids only contact one side of the reservoir formation due to the effects of gravity. Thus, all of the sides of the wellbore would not be contacted.
The use of cool coil tubing stimulation is also a problem because the tubing cannot reach the bottom of deep horizontal wells.
The March 2000 issue of SPE Drill & Completion, Volume 15, No. 1, reported on the use of manganese tetroxide with other additives in various formulations which proved to have negative effects on reservoir performance.
In SPE Publication No. 29409 (1995), a manganese tetroxide formulation is disclosed which requires a special stimulation method and, thus, is not suitable for all types of wells.
In SPE Publication No. 62729 (2000), a manganese tetroxide-containing formulation is presented in Table 4 which proved damaging to the reservoir. Also in the same paper at page 5, manganese tetroxide is employed with hematite and increases the formulation's density and improves its suspension. However, once again, special stimulation methods are needed in order to provide partial restoration of the permeability of the hydrocarbons to flow.
Also in SPE Publication No. 87127 (March 2004), tables 2 and 3 set forth two separate formulations wherein manganese tetroxide was employed along with other additives. Both of these formulations produced negative effects, which required subsequent stimulation.
It is an object of the present invention to provide a drilling fluid formulation containing manganese tetroxide wherein the reduction in the permeability of the reservoir formation to the flow of hydrocarbons is minimized.
Another object of the present invention is to provide a manganese tetroxide-based drilling fluid for use in wells which are difficult to stimulate.
It is still a further object of the present invention to provide a manganese tetroxide based wellbore drilling fluid which can be used effectively in horizontal wells, long-reached wells, and maximum reservoir wells, and which can provide hydrocarbon flows greater than 90% without requiring acid stimulation.
It is still a further object of the present invention to provide water-based manganese tetroxide drilling fluids that are useful over a wide pressure range, with good suspension and good solids content, and are 100% soluble in HCl.